Free radical polymerization reactions can be photoinitiated. The photoinitiator system can be based on various chemical approaches. For example, free radical polymerization reactions can be photoinitiated using a three-component photoinitiator system that includes an electron acceptor, an electron donor, and a sensitizing compound. The sensitizing compound usually absorbs actinic radiation resulting in the formation of an excited sensitizing compound. The electron donor can donate an electron to the excited sensitizing compound. That is, the sensitizing compound is reduced and the electron donor is oxidized. The reduced sensitizing compound is a radical anion that can donate an electron to an electron acceptor to yield an initiating free radical for the polymerization reaction. The initiating free radical is the reduced electron acceptor. In some instances of a three-component photoinitiator system, the oxidized electron donor is a radical species that also can function as an initiating free radical.
Other photoinitiator systems include a sensitizing compound and an electron donor but no electron acceptor. The sensitizing compound is usually an organic dye, an organic pigment, or an inorganic pigment. The sensitizing compound can absorb actinic radiation to form an exited sensitizing compound. The electron donor typically donates an electron to the excited sensitizing compound resulting in the oxidation of the electron donor. The oxidized electron donor is a radical species that functions as an initiating free radical for polymerization reactions.
In other free radical photopolymerization reactions, a salt such as a triarylsulfonium salt can generate an initiating free radical. Triarylsulfonium salts typically absorb ultraviolet radiation to form an excited triarylsulfonium ion. The excited triarylsulfonium ion can then photodegrade, abstract a hydrogen atom from another species present in the composition, and ultimately form a radical that can function as an initiating free radical for a polymerization reaction.